Intervertebral disc implant

ABSTRACT

An intervertebral disc implant having a body with a greater height than width comprised of a resilient material and an elongate cavity for receiving a key to maintain the spacing between the vertebrae adjacent an intervertebral disc when implanted into the space from which a portion of the disc is removed. To distribute and cushion against compression loads, and to mimic the normal kinematics of the intact, healthy intervertebral disc, the key is removed after the body is implanted into the disc space and a frame that both provides resistance to compression and tension loads and translates the axis of rotation of the spinal column anteriorally and posteriorally as the patient bends and rotates is inserted into the cavity in the implant body. The frame does not extend all the way into the cavity in the body and the portion of the cavity into which the frame does not extend is filled with a hydrogel.

This application in a continuation-in-part of co-pending applications Ser. No. 11/246,961, filed Oct. 7, 2005, entitled TOTAL ARTIFICIAL INTERVERTEBRAL DISC, Ser. No. 11/195,890, filed Aug. 2, 2005, entitled TOTAL ARTIFICIAL DISC, International Application No. PCT/US2005/009323, filed Mar. 19, 2005, entitled ROTATING, LOCKING, SPRING-LOADED ARTIFICIAL DISK, and Ser. No. 10/804,895, filed Mar. 19, 2004, entitled ROTATING, LOCKING, SPRING-LOADED ARTIFICIAL DISK.

BACKGROUND OF THE INVENTION

The present invention relates to an intervertebral disc implant for stabilizing two adjacent vertebrae that maintains the functions of the normal, healthy disc. More specifically, the present invention relates to a rectangularly-shaped disc implant that is expanded in the middle portion that is used as an alternative to spinal fusion.

Treatment of the damaged intervertebral disc, especially in the cervical and/or lumbar region of the spine, continues to be a challenging field of medicine. The classic treatment for a ruptured disc is diskectomy, i.e., removal of the disc from between the vertebrae. In this process, all or a portion of the intervertebral disc is removed, leaving a defect that may bother the patient throughout the rest of their life and compromising the normal interaction between disc and adjacent vertebrae. A procedure that is sometimes used as an alternative to diskectomy is to remove some or all of the disc and then fill the disc space with a bone graft, usually bone chips cut from the patient's iliac crest, or bone plug, bringing about fusion of the vertebrae above and below the disc, eliminating the empty space between the vertebrae.

Diskectomy with fusion is not ideal because the replaced bone does not have the function of the cartilaginous tissue of the disc, i.e. no cushioning effect, and has complications because of several factors. First, conventional bone plugs used to pack the disc space do not conform to the space of the disc because the disc bulges maximally in the center while the bone plug is generally cylindrically shaped and the disc space is wider in the middle and narrower at its anterior and posterior ends. For this reason, many commercially available bone plugs have just four points at which they contact the bodies of the adjacent vertebrae, i.e. two points at each of the front and back of the disc space. Second, access to the disc is from the side of the dorsal spine of the adjacent vertebrae, leaving a space that is “off-center” relative to the bodies of the adjacent vertebrae such that the stability of the implant is even more problematical than might be apparent from the limited contact resulting from the shape of the intervertebral space. Another complication is the possibility of infection or other conditions that may require removal of the implant. Also, if the bone pieces do not fuse, they may eventually extrude out of the disc space, pressuring the nerve roots. The most significant disadvantages of fusion, however, is that it eliminates all motion at the joint between the two vertebrae as well as the shock-absorbing/cushioning function of the disc.

Various prosthetic disc plugs, or implants, are disclosed in the art, but all are characterized by limitations of not conforming to the shape of the disc space, lack of stability when inserted off-center, inability to be removed, or other disadvantages. For instance, U.S. Pat. No. 4,863,476 (and its European counterpart, EP-A-0260044) describes an elongate, generally cylindrically-shaped body divided longitudinally into two portions having a cam device between the two portions for increasing the space between the two body portions once inserted into the disc space. However, because that device is cylindrical in shape such that the only contact points between the device and the vertebral bodies are at the front and back of the disc space, creating increased likelihood of instability, that device is generally unsuitable for use after partial diskectomy.

The art also discloses intervertebral disc prostheses such as are shown in U.S. Pat. Nos. 3,867,728, 4,309,777, 4,863,477, 4,932,969, Applicant's own Pat. No. 5,123,926, and French Patent Application No. 8816184 that may have more general contact with the adjacent discs, and spinal joint prostheses as described in U.S. Pat. No. 4,759,769, but which are not intended for use in fusion of the discs. The utility of such devices is also limited by a number of disadvantages, in particular, the same lack of cushioning described above in connection with prior art disc plugs and implants. Further, those implants and prostheses that attempt to address this cushioning problem have generally failed because they are not capable of supporting the load imposed upon them by the active post-surgical patient. Further, many prior implants and prostheses require removal of the disc. Removing the disc is not totally undesirable because removing the intervertebral disc does help prevent problems from recurrent disc herniation through the opening into the intervertebral disc space. However, as with all surgical procedures, it is desirable to utilize as much existing structure as possible and to minimize invasiveness. One reason it is desirable to retain as much of the original disc as possible is that if an implant subsequently fails, or if further surgical intervention is indicated for reasons such as infection, the only alternative that is generally available after removal of the intervertebral disc is fusion.

There is, therefore, a need for a device capable of stabilizing the vertebrae adjacent an intervertebral disc that overcomes the various disadvantages and limitations of known spinal fusion procedures and the disc plugs and implants that are used in such procedures, and it is an object of the present invention to provide apparatus and methods for meeting that need.

There is also a need for a device that can be implanted into the disc space in a procedure that decreases the likelihood of recurrent disc herniation and it is also an object of the present invention to provide apparatus and methods for meeting that need.

There is also a need for a device that mimics the function of the disc, in part by retaining as much of the undamaged disc as possible, that cooperates with the remaining portion of the disc to function in a manner similar to the normal, intact disc to provide the cushioning effect of the disc, and it is an object of the present invention to provide apparatus and methods for meeting that need.

There is also a need for a device that not only functions to provide the cushioning effect of the intervertebral disc but that also provides the opportunity for repairing the remaining portion of the disc, and it is an object of the present invention to provide apparatus and methods for meeting that need.

Another need that is apparent from the limitations and disadvantages of prior procedures, disc plugs, and prostheses is the need for a device that maintains the function of the healthy, intact intervertebral disc when implanted between adjacent vertebrae, is capable of being implanted in a surgical procedure that is minimally invasive, and that does not require removal of the entire intervertebral disc, and it is therefore also an object of the present invention to provide apparatus and methods for meeting that need.

Another need that is apparent from the limitations and disadvantages of prior procedures, disc plugs, and prostheses is the need for a device that works with the structure of the intervertebral disc space to maintain as much of the normal function of the disc as possible, and it is also an object of the present invention to provide apparatus and methods that combine the properties of cushioning (by utilizing the remaining portion of the disc), stability (by utilizing a monolithic, biconvex implant), shock absorption (by providing different cushioning characteristics in different portions of the disc space), and provide the opportunity to help reconstruct and/or prevent recurrent herniation of the remaining portion of the disc (by utilizing a hydrogel to fill gaps in the disc space and using known surgical repair techniques) thereby meeting that need.

Another need that is apparent is the need for a device that is capable of supporting the load imposed upon it when implanted in the disc space while also providing the cushioning function of the natural intervertebral disc and it is also an object of the present invention to provide apparatus and methods for meeting that need.

Another need that is apparent is a need for a frame for an intervertebral disc implant comprised of two spaced apart, substantially parallel arms, a bridge connecting the arms at one end, a “U”-shaped ear extending at approximately a right angle from the end of one of the arms opposite the bridge and having a hole formed therein for receiving a screw, and a “Y”-shaped ear extending at approximately a right angle from the end of one of the arms opposite the bridge having holes formed in both forks of the Y-shaped ear for receiving screws, the frame being comprised of a material that tends to return to its original shape after the frame is subjected to either a compression or tension load object of the present invention is to provide a frame for meeting that need.

Other objects, and the many advantages of the present invention, will be made clear to those skilled in the art in the following detailed description of several preferred embodiments of the present invention and the drawings appended hereto. Those skilled in the art will recognize, however, that the embodiments of the invention described herein are only examples provided for the purpose of describing the making and using of the present invention and that they are not the only embodiments of artificial discs that are constructed in accordance with the teachings of the present invention.

SUMMARY OF THE INVENTION

The present invention addresses the above-described problem by providing an intervertebral disc implant comprising an elongate body comprised of a resilient material having a cavity extending longitudinally into the body, the height of the body being greater than the width of the body, and a frame received within the cavity in the body comprised of two spaced apart, substantially parallel arms and a bridge connecting the arms at one end, the frame being comprised of a material that tends to return to its original shape after the frame is subjected to either a compression or tension load, the frame extending only part way into the cavity of the body in which it is received.

In another aspect, the present invention provides a method of mimicking the function of the intervertebral disc of the intact spinal column after removal of a portion or all of the intervertebral disc from between the two adjacent vertebrae comprising the steps of inserting a resilient body having a height greater than its width and a cavity formed therein with a key received in the cavity into the intervertebral disc space with the height of the body oriented substantially parallel to the longitudinal axis of the spinal column, removing the key from the cavity in the body after the body is inserted into the intervertebral disc space, and inserting a frame part way into the cavity in the body, the frame comprising first and second arms arms connected by a bridge at one end for providing resistance to flexion and/or extension of the spinal column, and filling the portion of the cavity in the body into which the frame does not extend with a hydrogel.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the figures, FIG. 1 shows a side elevational view of a presently preferred embodiment of an intervertebral disc implant constructed in accordance with the teachings of the present invention showing the key assembled to the implant for implantation in the intervertebral disc space.

FIG. 2 is an end view of the intervertebral disc implant of FIG. 1.

FIG. 3 is a perspective view of the intervertebral disc implant of FIG. 1 after removal of the key from the cavity in the body of the implant and insertion of the frame into the cavity.

FIG. 4 is a side elevational view of the invertebral disc implant of FIG. 3.

FIG. 5 is an end view of the intervertebral disc implant of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In more detail, FIG. 1 shows a presently preferred embodiment of an intervertebral disc implant constructed in accordance with the teachings of the present invention at reference numeral 10. Disc implant 10 is comprised of three components, each described in more detail below, implant body 12, key 14, and frame 16. Body 12 is preferably molded from a resilient, polymeric material. Although not limited to these materials, in the preferred embodiment, body 12 is molded from a biocompatible, viscoelastic polymer such as silicone, a urethane such as a polycarbonate urethane, or a polyurethane. As shown in FIGS. 1, 2, and 3, the body 12 is molded with a profile that approximates the shape of the normal intervertebral disc space with a height H greater than the width W (see FIG. 2); the top and bottom surfaces 36 of body 12 are arched so that the height of body 12 is greater in the center than at its ends. This shape of body 12 is referred to as being biconvex, e.g., both the top and bottom surfaces 36 of body 12 are convex in the anterior-posterior direction. Although shown in the figures as being configured for the use of the intervertebral disc implant 10 vertebrae in the lumbar region of the spine, those skilled in the art will recognize from this description that, with appropriate changes in size and configuration, the intervertebral disc implant of the present invention can also be utilized to advantage for replacement of a portion of a disc located in other portions of the spine.

Not only are the top and bottom surfaces 36 of body 12 convex in the anterior-posterior and side-to-side directions, but they are also provided with an integral metal strip 20 that may be textured or provided with a grooved surface to facilitate the ingrowth of bone onto the surfaces 36. In the preferred embodiment shown, the metal strips 20 are provided with structure for resisting anterior-posterior movement of body 12 once inserted into the disc space in the form of prongs 22 for biting into the cortical bone on the bearing surface of the adjacent vertebrae (not shown). In a second embodiment, the metal strips 20 affixed to the surfaces 36 of body 12 are covered with a porous or roughened titanium coating and perhaps even a layer of calcium phosphate for this purpose; other suitable coatings/surfaces are known in the art and include titanium wire mesh, plasma-sprayed titanium, porous cobalt-chromium and bioactive materials such as hydroxyapatite and the aforementioned calcium phosphate. This component of the artificial disc 10 of the present invention functions in a manner similar to the function of the cartilage of the normal, healthy disc to facilitate ingrowth of bone on the surfaces 36.

The implant body 12 is provided with an elongate cavity 18 for receiving a key 14 therein. In the preferred embodiment shown, the cavity 18 is formed with a portion near the opening into body 12 that is rectangularly-shaped (when viewed in cross-section) with dimensions that approximate the rectangular shape of key 14 and an enlarged portion deeper into body 12, both for a purpose described below. Because the body 12 is comprised of a resilient material that can be compressed, the key 14 is comprised of a relatively incompressible material such as stainless steel, titanium, or a polymer such as nylon or polycarbonate (or any other relatively imcompressible biocompatible material) to maintain the shape of the body 12 when inserted into the intervertebral disc space after removal of a portion of the intervertebral disc. Although not limited to this use, the intervertebral implant 10 of the present invention is optimally placed in the space from which a portion of the intervertebral disc has been removed at the centerline of the spinal column.

Frame 16 is shown in FIGS. 3, 4, and 5, and by reference to those figures, it can be seen that frame 16 is comprised of two spaced apart arms 24 connected at one end by a bridge 26. One or both of the ends 25 of the arms 24 opposite bridge 26 are provided with ears 28 having one or more holes 30 formed therein for receiving one or more screws (not shown) for securing frame 16 to the bodies of the vertebrae (not shown) adjacent the intervertebral disc space into which disc implant 10 is inserted. In the preferred embodiment shown, both ends of arms 24 are provided with ears 28, the ear 28A on the end of one arm 24A being shaped in the form of a Y” and having two holes 30 formed therein, the portion of ear 28A between the holes 30 being cutout at 32 to form the arms of the “U”-shaped ear 28A, and the ear 28B on the end of the other arm 24B being shaped in the form of an inverted “U” and having a single hole 30 therein. This arrangement of “Y” and inverted “U”-shaped ears 28A and 28B allows the use of the disc implant 10 of the present invention in the intervertebral disc spaces of successive segments of the spinal column. When secured to the body of the adjacent vertebra, the inverted “U”-shaped ear 28B of one artificial disc extends into the cutout portion 32 of the “Y”-shaped ear 28A secured to the body of that same vertebra.

As best shown in FIG. 4, when inserted into the cavity 18 in body 12 after the key 14 has been removed therefrom, frame 16 does not extend all the way into cavity 18. In other words, the bridge 26 of frame 16 is positioned only about half way into the cavity 18 in disc implant 12. As noted above, cavity 18 is formed of a portion near the opening into the cavity with a shape that approximates the rectangular cross-sectional shape of key 14, and as best shown in FIG. 4, frame 16 resides in that portion of the cavity near the opening into cavity 18. The dimensions of the portion of frame 16 that includes the arms 24 and bridge 26 are preferably approximately the same as the internal dimensions of this portion of cavity 18 (and coincidentally, the rectangular cross-sectional shape of key 14), although in one preferred embodiment, the dimensions of the portion of cavity 18 near the opening into cavity 18 is slightly smaller than the dimensions of the portion of frame 16 that includes the arms 24 and bridge 26 so as to help retain the frame 16 therein. In otherwords, it may be advantageous for the relative dimensions of the frame 16 and the opening into cavity 18 to be sized so that the frame 16 slightly stretches or expands the resilient material comprising implant body 12 when frame 16 is inserted into cavity 18. Similarly, the arms 24 of frame 16 may be angled slightly outwardly, or apart from each other, so that once the frame 16 is inserted into the cavity 18 in body 12, the resilience of the material comprising body 12, which may be stretched or expanded by insertion of the frame 16, tends to retain frame 16 in the opening into cavity 18. This slight angle of the arms 24 of frame 16, which is preferably less than about 5% from parallel, is exaggerated in the figures for purposes of illustration; in actual practice, the angle is small enough that it is appropriate to refer to the arms 24 as being substantially parallel to each other.

In the preferred embodiment, frame 16 is comprised of a material that tends to return to its original shape after the frame is subjected to either a compression or tension load. Materials that are characterized by this spring-like function when formed into the frame 16 include, but are not limited to stainless steel, titanium and titanium alloys, cobalt-chrome (Co—Cr) alloys, cobalt-chromium-molybdenum (Co—Cr—Mo), and medical grade (inert) polymeric plastics such as polyethylene, all as known in the art. In other words, when the implant body 12 is inserted into the intervertebral disc space and key 14 is removed and replaced by frame 16, body 12 is subjected to both compression and tension loads as the spine flexes and extends and as the patient moves during his/her normal daily routine. When subjected to these compression and tension loads, frame 16 deforms. Under compression, the ends 25 of the arms 24 opposite bridge 26 tend to move closer to each other and when in tension, the ends 25 of the arms 24 opposite bridge 26 tend to move further apart; in other words, the arms 24 of frame 16 deviate from their original spaced apart position (in the preferred embodiment shown, the two arms are substantially parallel, but those skilled in the art who have the benefit of this disclosure will recognize that the invention is not limited to a frame having parallel arms) when under compression or tension force. When the respective compression or tension force is relieved, the frame 16 tends to return to its original shape, i.e., the ends 25 of arms 24 opposite bridge 26 return to their original spaced relationship, and the arms therefore assume their original, spaced apart relationship. When subjected to loads in this manner, frame 16 acts as both a “backbone” and as a spring to help both bear compression loads and relieve tension loads in a manner that mimics normal disc function.

Note also that when the implant body 12 is inserted into the intervertebral disc space, the bridge 26 of frame 16 is positioned posteriorally relative to the ends 25 of arms 24 opposite bridge 26. The spring function of frame 16 is advantageous because, as the patient bends forward, the ends of arms 24 opposite bridge 26 are subjected to compression loads, and the further the patient bends, the more the material comprising frame 16 tends to resist the compression load, providing the spring function discussed above. As best shown in FIG. 4, the bridge 26 of frame 16 is provided with a projection 34 that extends between the arms 24 that functions to limit the bending of the arms 24, thereby limiting the flexure of the spinal column at the vertebrae adjacent the implant body 12. Further, biomechanical studies of normal, healthy spines have shown that the axis of rotation (the weight-bearing center of the intervertebral disc) translates anteriorally and posteriorally as the spine flexes and extends, and the variable resistance provided by this configuration and placement of frame 16 in the intervertebral disc space helps provide this normal front-to-back shift in the axis of rotation so that the disc implant of the present invention replicates that shifting in the axis of rotation.

As noted above, implant body 12 is provided with a cavity 18 and after positioning the implant body 12 in the space from which a portion of the intervertebral disc has been removed and removing the key 14 from cavity 18, the frame 16 is inserted into the cavity in place of the key. As is best shown in FIG. 4, frame 16 does not extend all of the way into the elongate cavity 18, leaving a space that in the preferred embodiment is at least partially filled with a hydrogel such as a polyvinyl alcohol (PVA), synthetic silk-elastin copolymers, polymethyl- or polyethylmethacrylate, polyethylene or polyacrylonitrile that absorbs water and increases in volume upon absorption of water, thereby functioning to maintain disc height in a manner similar to the manner in which the healthy disc maintains proper spacing between adjacent vertebrae. As best shown in FIGS. 3-5, the central projection 34 of the bridge 26 of body 12 is provided with a port 38 through which hydrogel is added (or removed) from the portion of the cavity 18 that is not occupied by frame 16. Port 38 is comprised of a channel that extends through the central projection 34 into the cavity 18 and the surgeon injects the hydrogel (or uses a syringe to remove hydrogel) as needed to confer the desired amount of initial disc height to the implanted body 12. Once the desired disc height is obtained, the port 38 is capped or plugged to prevent extrusion of the hydrogel contained within cavity 18. In an alternative embodiment, a one-way valve of a type known in the art may be utilized for this purpose. As described above, the cavity 18 is provided with a portion having expanded dimensions (as compared to the dimensions of the portion of cavity 18 proximate the opening into the cavity) for the purpose of allowing the injection of increased amounts of hydrogel for extra adjustability in obtaining the desired disc height. Some or all of the prongs 22 used to resist anterior-posterior movement of the implant body 12 in the disc space are provided with channels 40 communicating with cavity 18 for the purpose of allowing ingress and egress of fluid into and out of the hydrogel contained in cavity 18.

The positioning of frame 16 only part way into the cavity 18 in body 12 serves an additional purpose. The spring function of frame 16 provides resistance to compression and tension loads, but it also functions to allow anterior-posterior translation of the axis of rotation as the spine flexes so as to mimic the kinematics of the healthy disc. First, because of the resilient nature of the material comprising body 12, the portion of the material comprising body 12 that is positioned between the arms 24 of frame 16 and the bearing surfaces of the vertebral bodies of the adjacent vertebrae provides additional cushioning and resistance to the deformation of the frame 16 under extraordinary compression load. Second, because it extends only part way into the cavity 18 of body 12, the body 12 having the frame 16 positioned therein provides different amounts of resistance to compression load as the patient bends. The resistance to compression provided by the location of frame 16 part way into cavity 18 is greater as spinal flexion increases compared to the resistance to compression provided by the portion of implant body 12 in which the resistance to compression is provided by the material comprising body 12 and the hydrogel located in the portion of cavity 18 into which frame 16 does not extend. Of course the surgeon has the opportunity to fine-tune the amount of resistance as the spine flexes and/or extends by utilizing a frame with arms 24 of greater or shorter length so as to provide less or more resistance to flexure, respectively, and by adding or removing hydrogel in the portion of the cavity 18 into which frame 16 does not extend.

The healthy intervertebral disc includes three parts, the nucleus pulposus, annulus fibrosus, and cartilagenous endplate, each with separate functions, and all of which cooperate to provide motion, load bearing characteristics, and the other functions of the intact spinal column. The intervertebral disc implant 10 of the present invention provides corresponding parts, with their corresponding contribution to function, in the form of the implant body 12, which functions in a manner similar to the annulus fibrosus to maintain disc height, the frame-filled and hydrogel-filled cavity 18 which functions in a manner similar to the nucleus pulposus to distribute load and resist compression and tension, and the roughened surface of the metal strips 20 on surfaces 36, which functions in a manner similar to the cartilagenous endplate by providing a surface that facilitates ingrowth of the bone, thereby making the intervertebral implant of the present invention an integral part of the spinal column. As noted above, the surfaces of the metal strips 20 may be provided with prongs 22 or other structure for engaging the adjacent vertebrae to resist anterior-posterior movement of the implant 10 in the intervertebral disc space, but ingrowth of bone, facilitated by the roughened surface and/or surface coating on strips 20, is particularly advantageous in retaining the implant 10 in the disc space.

Those skilled in the art who have the benefit of this disclosure will recognize that certain changes can be made to the component parts of the apparatus of the present invention without changing the manner in which those parts function and/or interact to achieve their intended result. By way of example, those skilled in the art who have the benefit of this disclosure will recognize that the amount of resistance to compression and/or tension load provided by the frame 16 depends on such factors as the length of the arms 24 and the material comprising the frame 16 and that although it may be appropriate to implant an artificial disc constructed in accordance with the teachings of the present invention having a frame with a certain level of resistance to compression/tension load, it may be that an intervertebral disc including a frame with a different level of resistance to compression/tension load is better suited for implantation in another patient. It will also be recognized by those skilled in the art that to obtain desirable load resistance properties, it may be advantageous to increase the thickness of the central projection 34 so as to limit movement of the arms 24 toward each other under compression load or to make the implant body 12 from a combination of materials, with an embedded layer of material that is relatively incompressible or having a second set of resilience and/or load-bearing characteristics, or as a laminated “sandwich” of polyurethane and other material(s), each material adding a unique component to the load bearing characteristics of the implant body 12. All such changes, and others that will be clear to those skilled in the art from this description of the preferred embodiments of the invention, are intended to fall within the scope of the following, non-limiting claims. 

1. An intervertebral disc implant comprising an elongate body comprised of a resilient material having a cavity extending longitudinally into said body and a frame received within the cavity in said body comprised of two spaced apart arms with a bridge connecting the arms at one end, said frame being comprised of a material that tends to return to its original shape after said frame is subjected to either a compression or tension load, said frame extending only part way into the cavity of said body in which it is received.
 2. The intervertebral disc implant of claim 1 wherein said frame is provided with a port for receiving an injection of hydrogel therein.
 3. The intervertebral disc implant of claim 2 wherein said port is positioned so that the hydrogel injected into said port fills the portion of the cavity into which said frame does not extend.
 4. The intervertebral disc implant of claim 1 additionally comprising a metal strip attached to at least a portion of the surfaces of said body that bear against adjacent vertebrae when implanted into the intervertebral disc space, said metal strip being provided with structure for contacting the adjacent vertebrae to resist movement of said body relative to the adjacent vertebrae.
 5. The intervertebral disc implant of claim 4 wherein at least a portion of said metal strip is provided with a coating of or comprises porous or roughened titanium, calcium phosphate, titanium wire mesh, plasma-sprayed titanium, porous cobalt-chromium, or hydroxyapatite.
 6. The intervertebral disc implant of claim 1 additionally comprising a key positioned in the cavity in said body and extending substantially all the way into the cavity, said key being withdrawn from the cavity after said body is implanted into the space between two adjacent vertebrae and before said frame is inserted into the cavity in place of said key.
 7. The intervertebral disc implant of claim 1 additionally comprising a “U”-shaped ear extending at approximately a right angle from the end of one of the arms opposite the bridge and having a hole formed therein for receiving a screw for attaching said frame to a first adjacent vertebra and a “Y”-shaped ear extending at approximately a right angle from the end of the other arm opposite the bridge having holes formed in both forks of the Y-shaped ear for receiving screws for attaching said frame to a second adjacent vertebra.
 8. The intervertebral disc implant of claim 1 additionally comprising a projection extending from the bridge between the arms of said frame.
 9. The intervertebral disc implant of claim 1 wherein the arms of said frame are angled apart from each other.
 10. A method of mimicking the function of the intervertebral disc of the intact spinal column after removal of some or all of the intervertebral disc from between two adjacent vertebrae comprising the steps of: inserting a first resilient body having a cavity formed therein and a key received in the cavity into the intervertebral disc space with the height of the body oriented substantially parallel to the longitudinal axis of the spinal column; removing the key from the cavity after the body is inserted into the intervertebral disc space; inserting a frame part way into the cavity in the body, the frame comprising first and second arms spaced apart arms connected by a bridge at one end for providing resistance to flexion and/or extension of the spinal column; and filling the portion of the cavity in the body into which the frame does not extend with a hydrogel.
 11. The method of claim 10 additionally comprising attaching the frame to the adjacent first and second vertebrae.
 12. The method of claim 10 additionally comprising resisting anterior or posterior movement of the body in the intervertebral disc space.
 13. The method of claim 10 additionally comprising limiting the movement of the arms of the frame as the spinal column flexes.
 14. The method of claim 10 additionally comprising resisting relative movement between the frame and the implant body. 